1. Field of the Invention
The present invention relates to a temperature and a humidity sensing indicator used as a hygrometer for monitoring equilibrium relative humidity (ERH) of stacked paper sheets and the ambient relative humidity (ARH) of the surrounding air. The measurements are correlated on a table with the difference in relative humidity readings determining the conditions for printing on the paper.
2. Description of the Prior Art
The moisture content of paper influences its physical properties and produces dimensional variations which effect the printing conditions. It is important for the printer to know the temperature and humidity of the air surrounding the paper as well as the equilibrium relative humidity of the paper sheets or rolls. Present devices for measuring these properties include mechanical and electronic devices such as sword probes which can be inserted into the paper stacks to measure the temperature and relative humidity of the paper. These devices are quite expensive, cumbersome and do not provide a simple way to measure ambient relative humidity and for correlating the readings to determine the difference and the effect on printing conditions. Efforts to minimize the difference between ambient relative humidity and the paper ERH include the use of plastic covers for the paper to maintain the desired conditions before printing and temperature and humidity controls for the ambient air. However, there has been no simplified tabular guide to indicate a safe range of operating conditions and a range of allowable differences in moisture for different printing conditions.
Known methods for indicating temperature include liquid crystal devices which change color with temperature, such as described in U.S. Pat. Nos. 4,464,064 and 4,034,609, while humidity sensing devices include paper impregnated with inorganic salt compositions which change color with humidity, such as described in U.S. Pat. No. 4,150,570. These known devices however have not been utilized together to provide a simplified temperature and humidity sensor which can be employed to obtain both ambient relative humidity and equilibrium relative humidity readings for monitoring paper printing conditions.